The demand for smaller and lower cost components for consumer electronics has increasingly led to efforts to reduce the sizes of various microwave components. An example of such a component is a microwave directional coupler utilized in wireless terminals for monitoring transmitted power. In such applications, size and weight may be critical parameters.
A conventional microwave directional coupler utilizes two 50 ohm transmission lines, each having an electrical length of one quarter wavelength at the operating frequency. The spacing between the transmission lines is selected to provide the desired electromagnetic coupling. At an operating frequency of 1.95 GHz, the length of a conventional microstrip directional coupler is 19 millimeters (mm). This dimension is large in proportion to the overall package size of typical wireless terminals.
A directional coupler is a passive device which couples part of the transmission power by a known amount out through another port, often by using two transmission lines set close enough together such that energy passing through one is coupled to the other. The term “main line” refers to the main transmission line. On some directional couplers, the main line is designed for high power operation (large connectors), while the coupled port may use a small SMA (SubMiniature version A) connector. Usually the isolated port is terminated with an internal or external matched load (typically 50 ohms).
Physical considerations such as an internal load on the isolated port will limit port operation. The coupled output from the directional coupler can be used to obtain the information (i.e., frequency and power level) on the signal without interrupting the main power flow in the system. It should be recognized that the coupled response is periodic with frequency. For example, a ¼ coupled line coupler will have responses at n/4 where n is an odd integer.
Common properties desired for all directional couplers are wide operational bandwidth, high directivity, and a good impedance match at all ports when the other ports are terminated in matched loads.
Microstrip directional couplers having a capacitor or other reactive element connected between the two transmission lines are disclosed in U.S. Pat. Nos. 4,216,446 and 5,159,298. The capacitor or other reactive element is stated to improve the directivity of the directional coupler.
A directional coupler having a capacitor connected between transmission lines and shunt capacitors connected between each transmission line and ground is disclosed in U.S. Pat. No. 5,243,305. The capacitors are connected at the center of the transmission lines and are stated to increase the directivity of the directional coupler.
A capacitively compensated microstrip directional coupler is disclosed in U.S. Pat. No. 4,999,593. Reactive coupling networks are coupled between the transmission lines of the directional coupler at each end. Each reactive coupling network includes a first capacitor coupled between a common node and the first transmission line, a second capacitor coupled between the common node and the second transmission line, and a third capacitor coupled between the common node and ground. This interconnection however eliminates the independence between the transmission lines.
All known prior art microwave directional couplers have had one or more drawbacks, including but not limited to unacceptable physical size and a large number of compensation components. Accordingly, there is a need for improved microwave directional couplers.
Directional couplers as disclosed above are a well known element for radio frequency equipment. The directional coupler (a.k.a. a power sampler) allows a sample of a radio frequency signal, which is input at an input terminal and output at an output terminal, to be extracted from the input signal. Properly designed, the directional coupler can distinguish between a signal input at the input terminal and a signal input at the output terminal. This characteristic is of particular use in a radio frequency transmitter in which both the input signal and a signal which is reflected from a mismatched antenna can be independently monitored. One or the other or both of these signals can be utilized in a power control circuit to control the output power of the transmitter.
Another element well known in the output circuit of a transmitter is a harmonic filter, which is employed to reduce the energy coupled to an antenna at harmonic frequencies of the desired output signal. In a system which consists of a transmitter coupled to an antenna, the harmonic filter can be a relatively simple low pass filter. In a system where the transmitter must share the same antenna with other equipment, e.g., a companion receiver, the harmonic filter may take on a somewhat more complex configuration. For example, a bandpass filter which passes only a relatively narrow band of frequencies at which the transmitter is designed to operate while rejecting all other frequencies has been used in critical applications such as cellular radiotelephones. In order to achieve the lowest insertion loss within the smallest practical size, frequency resonant structures such as helical or coaxial resonators have been the choice of radio equipment designers. Unfortunately, resonant structures experience a reduction in their attenuation characteristics at frequencies which are approximately odd order harmonics of the passband frequency. Such a response is known as flyback. In order to overcome the flyback response, equipment designers have placed additional filtering in series with the resonant structure bandpass filter. One example of this additional filtering may be found in U.S. Pat. No. 5,023,866.
A radio equipment designer wishing to design high performance radio equipment may elect to employ a directional coupler, a resonant structure bandpass filter and an odd order harmonic flyback filter, but heretofore, has been constrained to use conventionally realized individual circuit elements. Such a configuration, with individual circuit elements, can experience potentially higher failure rates and dramatically increased size and cost of equipment.
Other prior art solutions lack the versatility desired by radio equipment designers and operators.
As illustrated in FIG. 1, the prior art of U.S. Pat. No. 5,212,815 discloses a transceiver utilizing a directional coupler. The radio transmitter 101, of conventional design for radio telephone use, is coupled to the input of directional coupler 103, the output of which is coupled to a conventional isolator 105. The isolator 105 reduces the amount of reflected power conveyed back to the transmitter 101 caused by impedance mismatches in bandpass filter 107 or the antenna 109. The directional coupler 103 provides a sample of the transmitter output signal which is attenuated and coupled from a forward power port to a power control circuit 115. However, the directional coupler 103 is not tunable, nor can the operation of the power sampler be decoupled from the operation of the stubs. Thus, attributes of the coupler and filter cannot be independently achieved.
As illustrated in FIG. 2, the prior art of U.S. Pat. No. 6,150,898 discloses an integrated component providing the function of both a conventional directional coupler 201 and a low-pass filter 202 having two attenuation poles 208 and 209 at a specified frequency band without changing the line length. Stub lines are connected to both ends of a main transmission line 205 of a directional coupler and the frequency of the attenuation poles is determined by fixed characteristics including impedance, terminating conditions and line length of the stub line. However, the prior art integrated component is a low pass filter and not a band reject filter and additionally, the lengths and impedance of the integrated components are not adjustable, i.e., they are manufactured for a set frequency and thus are not readily adaptable to allow for independent tuning of the coupler and filtering functions.
In view of the deficiencies of the prior art, it is an object of the present subject matter to obviate these deficiencies by presenting an independently tunable, combined coupler and filter.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.